How do I make the method return type generic?

java

generics

type-safety

method-overloading

byAnton Shumikhin·Aug 4, 2024

Extend your methods' versatility with generic type parameters for your return types:

public <T> T transform(T data) {
    return data;
}

Tell the method the type it should return:

String str = transform("text"); // This will return a String
Integer num = transform(123); // This will return an Integer

In this scenario, the compiler determines T based on the passed argument, providing type safety and flexibility.

Comprehensive guide to generic return types

Bounding your generics

To specifically dictate your return to a certain class or its subclasses, use bounded type parameters:

public <T extends Number> T numericTransform(T data) {
    // operate on numeric types here
}

This ensures you'll always get a Number type, providing an extra layer of compile-time safety.

Runtime control with Class parameters

To gain control over the return type at runtime, include a Class object as a parameter:

public <T> T create(Class<T> clazz) {
    return clazz.cast( /* Instantiate an object here, you magician you */ );
}

The type.cast() function allows for a safe cast to T, avoiding unchecked warnings and ClassCastException surprises.

The signature matters! don't forget subclasses

Give a clear generic signature when defining your method:

public interface Transformer {
    <T> T transform(T input);
}

Subclasses can implement this in a type-safe way:

public class StringTransformer implements Transformer {
    @Override
    public String transform(String input) {
        // Strings only, no numbers allowed! 🚫
    }
}

A combination of these techniques helps us maintain clean and safe code by avoiding excessive type casting!

A friend with all benefits: typesafe containers

Often when we're dealing with collections, we introduce the risk of ClassCastException. A typesafe heterogeneous container can come to the rescue:

public class BagOfTricks {
    private Map<Class<?>, Object> bag = new HashMap<>();

    public <T> void addTrick(Class<T> type, T instance) {
        bag.put(type, instance);
    }

    public <T> T getTrick(Class<T> type) {
        return type.cast(bag.get(type));
    }
}

Use type.cast() to avoid unsafe casts and ClassCastException exceptions.

All-around solutions in design

Shoutout to method overloading!

You might not always need to use generics, method overloading can sometimes offer a more clear and type-safe solution:

public Dog getDog() {
    return new Dog(); // Who let the dogs out? This method did!
}

public Cat getCat() {
    return new Cat(); // Purrfect for cat people
}